His academic work focused on social history and American social, political and economic elite and power structures, especially as it relates to immigration, ethnic conflict and national identity.

Fox's publications have been reprinted due to their prescient nature, including The Decline of Aristocracy in the Politics of New York (1919, repr. 1971); a biography of Herbert L. Osgood (his father-in-law); and Yankees and Yorkers (1940, 1989). With Arthur M. Schlesinger, Sr., he was editor of the “A History of American Life” series. The Completion of Independence, 1790–1830, (1944, repr. 1971, 1996).